Among the goals of this project are to describe age differences and changes in reasoning performance and to investigate psychological processes underlying such age-related performance. In a major longitudinal study of logical problem solving in men, cross-sectional age differences and changes over six years were found in both analysis, i.e., obtaining information, and synthesis, i.e., using information to reach a solution. In the study of individual models of complex concept learning problems, two of the models were validated using problems other than those on which the models were constructed. Each individual's model in micro-theory detailing the memory, inference, and decision processes, and the capacities of the memory stores used in sovling the problems. A micro-theory is constructed from eight problems during which the problem solver thinks aloud throughout each problem. Two models, one for an 18 year old man and the other for a 63 year old man, were successfully validated using several criterion measures of how well each model predicted the solution behavior on eight other problems. The processes identified in the two models were similar except for an additional mechanism in the model of the older man which attempts to compensate for his more limited memory capacity by representing the stored information more efficiently, thus reducing the cognitive strain of the task.